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Mutualisms in a changing world: an evolutionary perspective

That's the title of a review article recently published in Ecology Letters. Authors include Toby Kiers, who did a PhD with me a few years ago, and Judy Bronstein, who visited UC Davis when I was just starting to work on evolution of cooperation and told me about some key papers in the field.

"I use this term ["struggle for existence"] in a large and metaphorical sense including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny" -- Darwin

Climate change, pollution, hunting, and introduced species can have direct effects on endangered species, but what about indirect effects? For example, a wild plant species, growing alone, might produce fewer seeds when exposed to higher temperatures. But if higher temperature hurts the plant's competitors or pests enough, the resulting indirect decreases in competition or pest damage might outweigh direct negative effects. On the other hand, a plant that depends on animals for seed dispersal could suffer more seed predation, as shown below, if climate change or over-hunting reduces disperser numbers, even if climate change and hunting have no direct effect on the plant itself.
Coleoptera larva attacks the fruit of Iriartea deltoidea in western Amazonia. Over-hunting of seed dispersers has resulted in huge caches of undispersed seeds at parental trees, vulnerable to attack by various pests. Photo: Patricia Alvarez.

This week's paper adds evolution to the already complex problem of mutualism (cooperation between species) in changing environments. For example, what if the timing of flowering and the emergence of a pollinating insect both depend on temperature, but in different ways? Then climatic change may reduce pollination, at least in the short run. Both species will continue to evolve, however, which could bring them back into sync -- unless the pollinators switch to a different host whose flowering time fits their new schedule!

Switching partners is one of three possible evolutionary responses discussed in this week's paper. The other two are abandonment of mutualism, which won't necessarily lead to the extinction of either partner, and good interactions going bad. Examples of mutualisms that have been abandoned include plants evolving to use wind pollination instead of insect pollinators, and plants that don't form symbioses with mycorrhizal fungi. These examples aren't all the result of human impacts on natural ecosystems, but they illustrate the potential for such changes.

Examples given of positive interactions becoming negative include fungi living inside leaves that were once apparently beneficial become harmful to trees weakened by drought. Mycorrhizal fungi, which provide their plant hosts with phosphorus, apparently become more costly or less beneficial with overuse of fertilizer. Ants that defend trees from herbivores have become harmful with changes in herbivore populations. In most of these cases, it is not clear whether these negative changes result from evolution (genetic changes with a species), ecological changes (different species interacting), phenotypic plasticity (different behavior without a genetic change), or (most likely), some combination of these. Out of 179 studies analyzed, only 15 looked at evolutionary aspects. Maybe this paper will encourage those studying changing mutualisms to pay more attention to evolution.

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